the rules say delegates have to show to count. they didn't show.

Sanders had 78% of his delegates show (1662/2124) while Clinton had 98% of hers (1695/1722). Thus Sanders lost the delegate vote. The 64 delegates of his that were turned away weren't registered Democrats which is required to be a convention delegate. 6 were seated for the vote after they protested the ruling and confirmed they were registered and met all additional criteria.

now the leader decided to change the rules to allow them to ignore the rules without taking a vote on that decision.

You'll need to clarify which "leader" you're talking about. If you are talking about the convention chair no rule was changed (as far as I am aware). The at-large and PLEO delegates are, by rule, determined based on the results of the state convention (which Clinton won 50-49% or 1695/3357). As such the at large dplit was 4-3 and PLEO 3-2. My understanding is that the Sanders delegates (who, by numbers alone, were not in the majority) wanted the at-large and PLEO delegates to be assigned based on county convention results, which would have Sanders winning for the same reason he lost the state convention, Clinton delegates didn't show at the county conventions.

why bother to have rules if we don't have to follow them or if those in power can at their whim decide

From this it would seem you're talking about the chair as the 'leader' in your previous sentence. The facts, as I have seen them widely reported on both sides, don't line up. The Sanders delegates wanted to " ignore that rule because it affects those we want to win", not the Clinton delegates. I may have the rule vote wrong, there is plenty of they said/they said going on here.